There's a nice lengthy post of the zfs-discuss mailing list that covers how scrub works on a raidz vdev. The gist of it is:

• read data block, compute checksum
• read checksum from disk, compare to computed checksum
• compute parity for block, compute checksum for parity block
• read checksum for parity block from disk, compare to computed checksum

If compute checksums don't match the checksums on disk, then data block is rebuilt from parity, checksums are compared, and data written overtop of the bad data block.